1. Technical Field
This invention relates to a spindle motor having the conduction function and a recording disk driving apparatus having the spindle motor.
2. Description of the Related Art
With the recent increase in the recording density and improvement of the performance of the information storage device, the recording disk driving apparatus has been more and more reduced in size and increased more and more in rotational speed, while the field of application has widened even to the operation in an adverse environment under harsh temperature conditions.
The spindle motor for rotationally driving the recording disk such a magnetic disk includes a rotor assembly having a shaft and a stationary assembly with a sleeve fixed thereon. The shaft is arranged on the inner periphery of the sleeve, and a rotor hub for mounting the recording disk is fixed at the upper end of the shaft. The sleeve is fixed on the inner periphery of a housing, and rotatably supports the shaft through a fluid dynamic bearing. Also, the housing is fixed by such means as an adhesive on a bracket or the like.
The recording disk driving apparatus using the spindle motor includes a magnetic head for reading the data from the recording disk or writing the data in the recording disk.
The spindle motor used for the recording disk driving apparatus described above poses the problem of static electricity described below. Specifically, once the spindle motor is driven and the recording disk is rotated at high speed, friction occurs between the recording disk and the air, with the result that the recording disk is charged by static electricity, thereby causing a potential difference between the recording disk and the bracket. This in turn generates a potential difference between the recording disk and the magnetic head, and electric discharge is caused between the recording disk and the magnetic head, thereby often breaking the magnetic head.
Especially, the recently developed magnetic head of the recording disk driving apparatus, due to the increased capacity and high density of the recording disk, employs a magnetic head of magneto-resistance effect type (MR magnetic head or GMR magnetic head, for example). The MR magnetic head and the GMR magnetic head structurally include an element having a high current density and are mainly formed of a thin film. Without some preventive structure against the potential difference, therefore, breakage may result.
To obviate this problem, a conduction structure for connecting the rotor assembly and the stationary assembly electrically to each other is required to be interposed between the rotor assembly and the stationary assembly. In the prior art, the conduction structure described below has been proposed.
In the structure with the rotor assembly supported on the stationary assembly through the fluid dynamic bearing, for example, the electrical conduction can be secured between the rotor assembly and the stationary assembly by attaching conductivity to a lubricating fluid. Further, the housing and the bracket can be set to an equal potential by using a conductive adhesive for fixing the housing constituting the stationary assembly and the bracket to each other. As an alternative, a part of the bonded portion between the housing and the bracket is plastically deformed and coupled by metal to secure the conduction.
In the prior art, however, the housing and the bracket are often formed of different types of material having different coefficients of thermal expansion. Also, the coefficient of thermal expansion of the conductive adhesive is often different from those of the housing and the bracket. Depending on the temperature of the operating environment, therefore, the bonded joint may develop an excessive thermal stress causing a cracking or separation.
In such a case, the rotation of the rotor assembly of the spindle motor transmits the vibration, etc. of the rotor assembly to the sleeve through the fluid dynamic bearing, then from the sleeve to the housing, and is applied to the conductive adhesive for fixing the housing and the bracket to each other. Unless a sufficient amount of the conductive adhesive is coated between the housing and the bracket, therefore, the vibration stress and the thermal stress, etc. due to the temperature of the operating environment causes the cracking or separation of the conductive adhesive, which may be aggravated gradually. Once this phenomenon occurs in the conductive adhesive, the electrical conduction between the housing and the bracket is adversely affected.
Also, many conductive adhesives use silver as a main component and therefore, are high in price. By minimizing the amount of the conductive adhesive used as possible, therefore, the cost saving is often attempted. An excessively small amount of the conductive adhesive, however, would adversely affect the conduction between the housing and the bracket.